Stacked ceramic capacitor 1, which is an example of a multilayer ceramic electronic part, is shown in FIG. 1. External electrode 4 of multilayer ceramic capacitor 1 is generally formed using a calcination type conductive paste or a thermosetting conductive paste by the following method.
The first method is a method in which a calcination type conductive paste prepared by mixing, for example, conductive particles, such as Ag powder or Cu powder, and glass frit into a vehicle is applied to the leading side for internal electrode 3 of a multilayer ceramic composite material, and dried and then calcined at a high temperature of 500 to 900° C. to form external electrode 4.
The second method is a method in which a thermosetting conductive paste prepared by mixing conductive particles, such as Ag powder, into a thermosetting resin is applied to the leading side for internal electrode 3 of a multilayer ceramic composite material, and then thermally cured at a low temperature of 150 to 250° C. to form external electrode 4 (see, for example, patent document 1).
The third method is a method in which a thermosetting conductive paste prepared by mixing a heat decomposable organometal compound, such as silver acetate, and conductive particles, such as Ag powder, into a thermosetting resin is applied to the leading side for internal electrode 3 of a multilayer ceramic composite material, and then thermally cured at 350° C. to form external electrode 4 (see, for example, patent document 2).
The fourth method is a method in which a thermosetting conductive paste comprising a thermosetting resin, high melting-point conductive particles, and metal powder having a melting point of 300° C. or lower is applied to the leading side for internal electrode 3 of a multilayer ceramic composite material, and then thermally cured at a low temperature of 80 to 400° C. to form external electrode 4 (see, for example, patent document 3).
In any methods, for improving the bonding strength of the resultant capacitor device when mounted on, e.g., a substrate by soldering, the surface of the electrode layer is optionally plated 5. For example, the surface of the external electrode is plated with Ni by electroplating in, e.g., a Watts bath, and then further subjected to solder plating or Sn plating by electroplating.
However, in the capacitor having an external electrode obtained by the first method, the glass frit component of the conductive paste diffuses into the capacitor device during the high-temperature calcination, and therefore a problem occurs in that, e.g., cracks are caused when mounting the capacitor on a substrate by soldering. Further, the plating solution penetrates the sintered material during the plating, so that the designed electrostatic capacity cannot be obtained or deterioration of the insulation resistance occurs, leading to a problem of the reliability of capacitor performance.
On the other hand, the capacitor having an external electrode obtained by the second method is free of the above problems caused when mounting the capacitor on a substrate or in plating. However, the curing temperature for the paste is low, and solid phase diffusion does not proceed between metals, i.e., the conductive particles, e.g., Ag powder contained in the conductive paste and the internal electrode, and hence the internal and external electrodes are not satisfactorily bonded together, so that desired electric properties including the designed electrostatic capacity cannot be obtained and the reliability is poor.
The capacitor having an external electrode obtained by the third method has disadvantages in that the silver acetate and amine contained in the paste causes the pot life of the paste to be shortened and deterioration of insulation occurs due to humidity aging.
Further, in the capacitor having an external electrode obtained by the fourth method, the solder reflow temperature in mounting of an electronic part on a substrate is increased for achieving lead free soldering due to the recent lead problems, and the increase of the temperature possibly melts again the low melting-point metal powder to cause the solder to burst.    Patent document 1: Japanese Unexamined Patent Publication No. Hei 6-267784    Patent document 2: Japanese Unexamined Patent Publication No. 2000-182883    Patent document 3: International Publication No. 2004/053901